Process for the preparation of trimethyl metal compounds

ABSTRACT

Process for the preparation of a trimethyl metal compound with the formula M(CH3)3, said process comprising the step of reacting a trialkyl metal compound of the formula M(R)3 with trimethyl aluminium [AI(CH3)3] to form said trimethyl metal compound with the formula M(CH3)3, wherein M is selected from the group consisting of Ga and In, and R is a linear or branched alkyl group with 2 to 8 carbon atoms.

The present invention relates to a method for producing trimethyl metal compounds selected from trimethyl gallium and trimethyl indium.

With the advancement of mobile phones and optical communication technologies, demand is rapidly growing for compound semiconductors for use in high speed electronic devices such as high electron mobility transistors (HEMTs), heterojunction bipolar transistors (HBTs), semiconductor lasers, optical devices such as white and blue super high-intensity LEDs, and other applications.

In general, alkyl derivatives of group 12 and group 13 metals, and in particular the methyl or ethyl derivatives, are often used as metalorganic precursors for compound semiconductors. A great demand exists for, in particular, trimethyl gallium for the production of compound semiconductors by MOCVD with group 15 elements, such as nitrogen, arsenic, and the like.

Trimethyl gallium (TMG) is conventionally prepared by reacting a gallium trihalide (e.g. gallium trichloride) with trimethyl aluminium (TMAL). According to this reaction, the production of one mole of TMG requires the use of 3 moles of TMAL:

GaCl₃+3Al(CH₃)₃→Ga(CH₃)₃+3Al(CH₃)₂Cl

Trimethyl indium can be produced in comparable manner.

TMAL is considerably more expensive than other alkyl aluminum compounds, such as triethyl aluminium and dimethyl aluminium chloride.

With three moles of TMAL being required for the production of only one mole of TMG, it will be clear that the TMG production costs are heavily determined by the TMAL price.

The object of the present invention is therefore to provide a process for the production of trimethyl metal compounds, in particular trimethyl gallium and trimethyl indium, that requires significantly less trimethyl aluminium.

This object has been achieved by the process of the present invention, which relates to the preparation of a trimethyl metal compound with the formula M(CH₃)₃, comprising the step of reacting a trialkyl metal compound of the formula M(R)₃ with trimethyl aluminium [Al(CH₃)₃] to form said trimethyl metal compound with the formula M(CH₃)₃,

wherein M is selected from the group consisting of Ga and In, and R is a linear or branched alkyl group with 2 to 8 carbon atoms.

As will be illustrated below, this process requires only one mole of TMAL per mole of produced trimethyl metal compound.

In a preferred embodiment, M is Ga.

Alkyl group R is preferably selected from ethyl and linear or branched propyl and butyl groups, including n-propyl, isopropyl, n-butyl, isobutyl, and tert-butyl. More preferably, the alkyl group is selected from ethyl, n-propyl, n-butyl, and isobutyl. Most preferably, the alkyl group R is ethyl.

According to the present process, a trialkyl metal compound M(R)₃ is reacted with trimethyl aluminium (TMAL) to form the trimethyl metal compound M(CH₃)₃. Most preferably, the process involves the reaction between triethyl gallium (TEG) and trimethyl aluminium (TMAL) to form trimethyl gallium (TMG). The side product is triethyl aluminium (TEAL):

Ga(CH₂CH₃)₃+Al(CH₃)₃→Ga(CH₃)₃+Al(CH₂CH₃)₃

Apart from the advantage that this process requires only one mole of expensive TMAL per mole of produced trimethyl metal compound, the additional advantage of this process is that the trialkyl aluminium Al(R)₃ that is formed as a side-product can be used in the preparation of trialkyl metal M(R)₃. The latter preparation involves the reaction between a metal trihalide MX₃ (X is Cl, Br, or I, more preferably Cl or Br, and most preferably Cl) and a trialkyl aluminium Al(R)₃ to form a trialkyl metal compound M(R)₃ and a dialkyl aluminium halide Al(R)₂X.

Hence, the trialkyl aluminium Al(R)₃ that is formed in the reaction towards trimethyl metal compound M(CH₃)₃ can be re-cycled to the preparation of the trialkyl metal compound M(R)₃.

This reaction is preferably conducted under inert (e.g. nitrogen) atmosphere at a temperature in the range 0-280° C., preferably 25-250° C., most preferably 50-175° C. The temperature can be kept constant during the reaction, but may also gradually rise.

The reaction can be performed at atmospheric pressure or lower pressures. At lower pressures, lower temperatures may be applied.

TMAL is preferably added to the trialkyl metal compound M(R)₃ in a slight excess compared to the theoretical molar ratio of 1:1. This excess is preferably 0-50 mol %, more preferably 0-25 mol %, and most preferably at most 0-10 mol %. The molar ratio TMAL:M(R)₃ is therefore preferably 1.0:1-1.5:1 , more preferably 1.0:1-1.3:1, and most preferably 1.0:1-1.1:1.

If desired, KF can be added to the reaction mixture in order to decompose any undesired aluminium complexes.

The reaction is carried out by introducing the trialkyl metal compound M(R)₃, trimethyl aluminium (TMAL), and optionally a solvent into a reaction vessel under inert gas atmosphere. These compounds can be added in any form and in any order.

Examples of suitable solvents are saturated aliphatic hydrocarbons like pentane, hexane, heptane, octane, nonane, decane, undecane, and dodecane; saturated alicyclic hydrocarbons like cyclohexane and cycloheptane; and aromatic hydrocarbons like toluene, xylene, trimethylbenzene, ethylbenzene, ethyltoluene, and indene. Preferred solvents are those that are easily separable from the resulting trimethyl metal compound, more preferably by having a boiling point that differs significantly from that of the trimethyl metal compound.

The desired product, i.e. the trimethyl metal compound that is formed in step (ii) can be isolated from the reaction mixture by distillation (optionally under vacuum) or crystallisation.

In a preferred embodiment, the reaction described below is preceded by a step that involves the preparation of the trialkyl metal compound M(R)₃. This preparation involves the reaction between a metal trihalide MX₃ and a trialkyl aluminium of the formula Al(R)₃ to form a trialkyl metal compound of the formula M(R)₃ and a dialkyl aluminium halide of the formula Al(R)₂X.

This then leads to a process for the preparation of a trimethyl metal compound with the formula M(CH₃)₃ comprising the steps of

-   (i) reacting a metal trihalide MX₃ with a trialkyl aluminium of the     formula Al(R)₃ to form a trialkyl metal compound of the formula     M(R)₃ and a dialkyl aluminium halide of the formula Al(R)₂X, and -   (ii) reacting said trialkyl metal compound of the formula M(R)₃ with     trimethyl aluminium (Al(CH₃)₃) to form the trimethyl metal compound     with the formula M(CH₃)₃,     wherein M is selected from the group consisting of Ga and In, -   X is a halogen, and -   R is a linear or branched alkyl group with 2 to 8 carbon atoms.

Again, M is preferably Ga and the halide X is preferably Cl, Br, or I, more preferably Cl or Br, and most preferably Cl. The metal trihalide is most preferably gallium trichloride.

The reaction of step (i) is preferably conducted under inert (e.g. nitrogen) atmosphere at a temperature in the range 0-280° C., preferably 25-250° C., most preferably 50-175° C.

The temperature can be kept constant during the reaction, but may also gradually rise.

The reaction of step (i) is carried out by introducing the metal trihalide and the trialkyl aluminium compound Al(R)₃, and optionally a solvent into a reaction vessel under inert gas atmosphere. These compounds can be added in any form and in any order.

Compounds that are solid under the addition conditions (e.g. indium trichoride or gallium trichloride) may be added as such, but may also be added to the reactor dissolved in a solvent or in molten form. Addition as solution is especially preferred if the process is conducted in continuous manner. Examples of suitable solvents are saturated aliphatic hydrocarbons like pentane, hexane, heptane, octane, nonane, decane, undecane, and dodecane; saturated alicyclic hydrocarbons like cyclohexane and cycloheptane; and aromatic hydrocarbons like toluene, xylene, trimethylbenzene, ethylbenzene, ethyltoluene, and indene. Preferred solvents are those that are easily separable from the resulting trimethyl metal compound, more preferably by having a boiling point that differs significantly from that of the trimethyl metal compound.

The reaction of step (i) most preferably involves the reaction between gallium trichloride and triethyl aluminium (TEAL) to form triethyl gallium (TEG) and diethyl aluminium chloride (DEAC):

GaCl₃+3Al(CH₂CH₃)₃→Ga(CH₂CH₃)₃+3Al(CH₂CH₃)₂Cl

The trialkyl aluminium Al(R)₃ is preferably added to the metal trihalide MX₃ in a slight excess compared to the theoretical molar ratio of 3:1.

This excess is preferably 0-10 mol %, more preferably 0-5 mol %, and most preferably at most 0-3 mol %. The molar ratio Al(R)₃:MX₃ is therefore preferably 3.0:1-3.3:1, more preferably 3.0:1-3.2:1, and most preferably 3.0:1-3.1:1.

The trialkyl metal compound that is formed in step (i) may be separated from the reaction mixture—in particular from the side product dialkyl aluminium halide—by distillation (optionally under vacuum) or crystallisation, for further use in step (ii).

The process as described above can be performed batch-wise but also continuously.

As mentioned above, the trialkyl aluminium Al(R)₃ side product that is formed may be re-cycled for use in step (i).

In an alternative embodiment, both reactions—i.e. the reaction towards the trialkyl metal compound M(R)₃ [step (i)] and the subsequent reaction towards the trimethyl metal compound [step (ii)]—may be performed in one single reactor (one pot reaction) by adding the required amounts of metal trihalide MX₃, trialkyl aluminium Al(R)₃, and trimethyl aluminium to a reactor and isolating the produced M(CH₃)₃ by distillation or crystallisation.

The trimethyl gallium and trimethyl indium obtained by the process of the present invention can be suitably used for the preparation of semiconductor devices, e.g. gallium nitride-based semiconductors.

EXAMPLES

All experiments were carried out in a glovebox with nitrogen as a protective gas.

Example 1 Synthesis Triethyl Gallium (TEG) from Gallium Trichloride and Triethylaluminum (TEAL)

TEAL (635 g) was heated to 70° C. in a 2-liter reaction vessel. A solution of 246 g GaCl₃ in 400 ml absolute n-hexane was dropwise added, while stirring, over a period of 2-3 hours. n-Hexane was distilled off at normal pressure. Subsequently, the TEG was distilled—under reduced pressure—from the higher boiling DEAC and excess TEAL.

Synthesis of Trimethyl Gallium (TMG) from TEG and Trimethyl Aluminium (TMAI)

14.60 g (0.093 mol, 1.0 eq.) of the TEG produced above and 8.00 g (0.111 mol, 1.2 eq.) TMAL (98.5%, AkzoNobel) were poured into a 50 ml two-neck round-bottom flask equipped with a distillation column, stirrer and a thermocouple. The mixture was first stirred at 20° C. and after complete homogenization the mixture was gradually heated to 140° C. The distillate was collected into a cooled receiving flask (−5° C.) with a top temperature of 56-61° C. TMG was isolated in a 92% yield (9.82 g, 0.085 mol) with a purity of 97-98% based on ¹H NMR analysis.

Example 2 One-Pot Synthesis of Trimethyl Gallium (TMG) from Gallium Trichloride, Triethylaluminum (TEAL), and Trimethyl Aluminium (TMAL)

17.60 g (0.100 mol, 1.0 eq.) of GaCl₃ (ex MCP) were poured into a 50 ml two-neck round-bottom flask equipped with a distillation column, stirrer and a thermocouple. 22.80 g (0.200 mol, 2 eq.) TEAL (>95% AkzoNobel) and 7.90 g (0.110 mol, 1.1 eq.) TMAL (98.5%, AkzoNobel) were mixed together and subsequently added dropwise to GaCl₃. The mixture was gradually heated to 160° C. The distillate was collected into a cooled receiving flask (−5° C.) with a top temperature of 56-61° C. TMG was isolated in a 78.1% yield (9.08 g, 0.079 mol) with a purity of 98% based on ¹H NMR analysis. 

1. Process for the preparation of a trimethyl metal compound with the formula M(CH₃)₃, said process comprising the step of reacting a trialkyl metal compound of the formula M(R)₃ with trimethyl aluminium [Al(CH₃)₃] to form said trimethyl metal compound with the formula M(CH₃)₃, wherein M is selected from the group consisting of Ga and In, and R is a linear or branched alkyl group with 2 to 8 carbon atoms.
 2. Process according to claim 1 wherein said step is preceded by a step that involves the production of said trialkyl metal compound of the formula M(R)₃ by reacting a metal trihalide of the formula MX₃ with a trialkyl aluminium of the formula Al(R)₃ to form said trialkyl metal compound of the formula M(R)₃ and a dialkyl aluminium halide of the formula Al(R)₂X.
 3. Process according to claim 2 wherein the process is conducted as a one-pot reaction, in which the metal trihalide MX₃, the trialkyl aluminium Al(R)₃, and trimethyl aluminium are added to a reactor to form the trimethyl metal compound M(CH₃)₃.
 4. Process according to claim 2, wherein the produced trialkyl metal compound M(R)₃ is isolated from the reaction mixture by distillation or crystallisation prior to its reaction with trimethyl aluminium (Al(CH₃)₃).
 5. Process according to claim 1 wherein the reaction is performed in continuous mode.
 6. Process according to claim 1 wherein M is Ga.
 7. Process according to claim 1 wherein R is selected from the group consisting of ethyl and linear or branched propyl and butyl groups.
 8. Process according to claim 7 wherein R is selected from the group consisting of ethyl, n-propyl, n-butyl, and isobutyl.
 9. Process according to claim 7 wherein R is ethyl.
 10. Process according to claim 1 wherein the trialkyl metal compound of the formula M(R)₃ is reacted with trimethyl aluminium (Al(CH₃)₃) to form the trimethyl metal compound with the formula M(CH₃)₃ and trialkyl aluminium with the formula Al(R)₃.
 11. Process according to claim 10 wherein the produced trialkyl aluminium is re-used for the production of the trialkyl metal compound M(R)₃ in accordance with claim
 2. 12. Process according to claim 1 wherein the produced trimethyl metal compound M(CH₃)₃ is isolated from the reaction mixture by distillation or crystallisation. 